If you have an advanced system key with limited features, you essentially cannot create Trunking System definitions in your codeplug. One of the options on the ASTRO25 box on the General tab can only be used if you have an unlimited key; if you try with a limited key, you end up in an endless loop of invalid fields (between the Home System ID and Home WACN ID). Admittedly this was for a multi-SystemID Harris system, so it may be a case that affects a narrow group of users.
What the eventual solution ended up being was that we had to email our codeplugs to the main system administrators, who obviously have the ability to issue themselves unlimited access keys. They built Trunking System definitions and then emailed us back the codeplugs. Those of us who have limited keys have to guard these codeplugs with our lives; if we touch the parameters in that part of the General tab in any way, we'll get into the same endless loop of invalid fields and the Trunking System will become unusable again.
In my opinion this is a flaw in the way ASTRO25 CPS is designed. There should not be such a restriction (this field can only be used if you have an unlimited key) in the software. The limitation isn't present in APX CPS; I can (and have) created Trunking System definitions with the exact same limited ASK, all day long, in APX CPS.
Motorola seems uninterested in fixing this flaw in ASTRO25 CPS. I understand that it's past EOL, but as you say, there are thousands upon thousands of radios out in the wild that depend on that software. It would be in Moto's best interest to issue a fix and keep their customers. If we weren't willing to accept the workaround of finding an unlimited key to do our programming (for example, if we had sensitive data in our codeplug for other systems, that we weren't allowed to let the unlimited key-holder to see), Moto's only other solution - "buy an APX" - would not be something some of us could easily do.